1. Field of the Invention
The present invention relates to semiconductor device technology, more specifically to a packaging assembly and a method of assembling a packaging assembly using soldering technology.
2. Description of the Related Art
Semiconductor integrated circuits such as LSI have achieved higher levels of integration in recent years. Semiconductor devices themselves are shrinking in geometrical size, increasing in the degree of on-chip integration, and number of pin-counts are increasing. As for the surface-mount package (SMP), flip chip bonding technology which connects a semiconductor chip and a circuit board with bumps has been employed widely. As for the flip chip bonding technology, an encapsulating resin is applied on a surface of a circuit board having bumps on the surface. Next, bumps formed on an element side of a semiconductor chip and the bumps formed on the circuit board are mated and contacted to each other. Furthermore, the circuit board and the semiconductor chip are heated around 150° C. during a reflow process, and oxide films and alien substances contained in the bumps are removed by the resin which serves as a flux. Then, bumps of the circuit board and the semiconductor chip are melted and connected during heating process at 200° C. After that, bumps and the resin are hardened completely in the curing process.
In the SMP assembling process, solder bumps made of solder paste are often used as bump electrodes. However, recently, it has been pointed out that the outflow of lead from electronic products dumped onto reclaimed land pollutes underground water. Thus, throughout the world, manufacturers are changing the Sn—Pb eutectic, used for mounting semiconductor chips or printed circuit boards, to lead-free solder alloys.
Material examples of lead-free solder alloys, responding to an environmental problem, are tin-silver (Sn—Ag) solder and tin-zinc (Sn—Zn) solder. However, for lead-free solder alloys such as Sn—Ag solder, the melting temperature is generally higher than that of the conventional eutectic alloy. Therefore, lead-free solder alloys having higher melting temperatures have to be reflowed at a relatively high temperature of approximately 200° C. However, when reflow is performed at high temperature conditions, strong thermal stresses are applied to semiconductor chips and mounting bases, and an aggravation of coplanarity and a fall in reliability occurs. Moreover, when organic materials are employed as a circuit board, a gas is generated from the circuit board and that invades into an underfill resin by reflowing at a high temperature of more than 200° C. On the other hand, while a curing reaction advances for the underfill resin, the viscosity of the underfill resin rises. As a result, the gas in the underfill resin remains as a void without being ejected outside of the underfill resin. Furthermore, since the heat shrinkage rate of underfill resin also increases by reflowing, thermal stresses occur to the electrodes on the semiconductor element side, and cracks in the electrodes arise.
Meanwhile, since recent microprocessors process huge quantities of information at high speed, there have been problems with the resistance of wires interconnecting transistors, and the capacitances of insulators between interconnect wires. For example, wire materials are changing from aluminum (Al) to copper (Cu) having a high electrical conductivity, and insulators are changing from silicon oxide films to materials having low dielectric constants. However, such materials used in recent electronic devices are generally weak in mechanical strength. In particular, low dielectric constant films (hereinafter called low-k films) used as insulators on semiconductor chips are significantly weak in mechanical strength and in adhesion intensity because of their porous structures necessary to ensure low dielectric constants. Therefore, when reflowing to electrodes is performed using a lead-free solder at a high melting temperature, strong thermal stresses also occur in the low-k films within the semiconductor chip. Furthermore, the low-k films disposed just under the solder electrodes may be damaged by the heat and the adhesive strength between the semiconductor chip and the mounting base is also decreased.